1. Field of the Invention
This invention relates to an image processing apparatus, such as an image processing apparatus having a function for detecting a specific image, such as that of a bank note or security.
2. Description of the Prior Art
Apparatus of this type that have recently been developed feature a large number of tones and a high-image quality in which an image signal expressing one pixel is composed of a large number of bits. Owing to greater sophistication of image processing and an improvement in recording density, even a binary printer is capable of providing a high picture quality approaching that of a multivalued printer.
However, the recent improvements in the picture quality of copying machines which now have a color capability have been accompanied by the fear of counterfeiting, in which specific originals such as bank notes and securities, which are not meant to be copied, are duplicated at such a high picture quality that the copies are almost indistinguishable from the originals.
Thus, a serious shortcoming in the prior art mentioned above is that effective measures for preventing counterfeiting have not been developed for binary printers in which one pixel is expressed by one bit (in the case of color, by the three bits, namely R, G, B bits, or by four bits, namely C, M, Y and Bk bits).